Sound only under the car (5)
I believe the answer is:
audio
'sound' is the definition.
(synonyms)
'only under the car' is the wordplay.
'only' becomes 'o' (this might be a standard abbreviation of which I'm unaware).
'under' is a charade indicator (letters next to each other).
'the car' becomes 'audi' (audi is a german car brand).
'o' after 'audi' is 'AUDIO'.
Can you help me to learn more?
(Other definitions for audio that I've seen before include "Sound system" , "Prefix indicating sound or word" , "type of reproduction" , "Sound broadcast" , "Prefix signifying sound or hearing" .)